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Vic Santos · 06-16-2010, 04:18 PM

Bill, in my last response I suggested tightening/closing the air valve and testing different hangers and secondary rod shapes. Here are the reasons.

The launch is difficult to sort out using a Q-Jet and every application is different. The first thing that happens when you let go of the button is "G" loads are applied to the carb; up to 2 1/2 G's depending on power and weight. The fuel is pushed back in the bowl and wants to stack at 45 degrees for 1 G and 63 degrees for 2 G's. It doesn't really happen this bad because of all the stuff in the bowl but you get the idea. As fuel is pushed back in the bowl it raises the float and momentarily SHUTS the inlet valve until enough fuel is consumed to lower the float and allow more in ( approx 0.3 sec ). In my car it then takes 1.1 sec before the bowl refills and the float shuts off fuel again ( about when I land from a wheel stand ). Fuel flow does not become steady until 3.5 sec into the run. At the point where the inlet valve reopens it is critical to have a fuel system with HIGH FLOW AT PRESSURE otherwise you will go lean.

( Talking about pressure I've tested 5 fuel gages and a transducer with a 183" column of water ( 6.61 psi ) and none read the same. The worst was off by 1.5 psi! )

In my testing I've found the amount of fuel IN the bowl is critical to the launch. To control this I've made the bowl as big as possible and I've changed float level until I got the best EGT & G level traces and the best 330 ft ET. ( many people feel the higher the fuel level the better- not based on my testing )

The fact that the fuel is stacked at the back of the bowl causes another problem-- the engine needs a leaner rod during the launch than at the other end of the track where the fuel level has stabilized at a lower level. The end result is the secondary rod is usually a compomise based on 1/4 mi ET. This usually results in the rod being too rich at the launch and too lean at the top end. This can be offset, somewhat, by using leaner hangers and short tip sec rods to lean the mixture during the first second when the air valve is opening. This can only happen when the air valve is closed when on the two-step. The only way I've been able to science this out is to make sure the air valve is closed when on the two-step. When the air valve is open on the two-step you have a 750 cfm carb wide open at a low RPM and you have to do lots of other things to richen the mixture since there is very little carb signal

If the air valve setting is too far open you will have to run richer rods and jets to offset the reduced carb signal and the launch will be WAY too rich.

Once you get close to the right set up you very seldom have to change primary jets or secondary rods more than one size and change only the hanger.

I know there are very succesful foot brake cars and my hat is off to them. I tried it for 5 years and couldn't get it right.

06-16-2010, 04:18 PM	#20
Vic Santos Junior Member Join Date: Dec 2008 Posts: 27 Likes: 0 Liked 14 Times in 2 Posts	Re: bog Bill, in my last response I suggested tightening/closing the air valve and testing different hangers and secondary rod shapes. Here are the reasons. The launch is difficult to sort out using a Q-Jet and every application is different. The first thing that happens when you let go of the button is "G" loads are applied to the carb; up to 2 1/2 G's depending on power and weight. The fuel is pushed back in the bowl and wants to stack at 45 degrees for 1 G and 63 degrees for 2 G's. It doesn't really happen this bad because of all the stuff in the bowl but you get the idea. As fuel is pushed back in the bowl it raises the float and momentarily SHUTS the inlet valve until enough fuel is consumed to lower the float and allow more in ( approx 0.3 sec ). In my car it then takes 1.1 sec before the bowl refills and the float shuts off fuel again ( about when I land from a wheel stand ). Fuel flow does not become steady until 3.5 sec into the run. At the point where the inlet valve reopens it is critical to have a fuel system with HIGH FLOW AT PRESSURE otherwise you will go lean. ( Talking about pressure I've tested 5 fuel gages and a transducer with a 183" column of water ( 6.61 psi ) and none read the same. The worst was off by 1.5 psi! ) In my testing I've found the amount of fuel IN the bowl is critical to the launch. To control this I've made the bowl as big as possible and I've changed float level until I got the best EGT & G level traces and the best 330 ft ET. ( many people feel the higher the fuel level the better- not based on my testing ) The fact that the fuel is stacked at the back of the bowl causes another problem-- the engine needs a leaner rod during the launch than at the other end of the track where the fuel level has stabilized at a lower level. The end result is the secondary rod is usually a compomise based on 1/4 mi ET. This usually results in the rod being too rich at the launch and too lean at the top end. This can be offset, somewhat, by using leaner hangers and short tip sec rods to lean the mixture during the first second when the air valve is opening. This can only happen when the air valve is closed when on the two-step. The only way I've been able to science this out is to make sure the air valve is closed when on the two-step. When the air valve is open on the two-step you have a 750 cfm carb wide open at a low RPM and you have to do lots of other things to richen the mixture since there is very little carb signal If the air valve setting is too far open you will have to run richer rods and jets to offset the reduced carb signal and the launch will be WAY too rich. Once you get close to the right set up you very seldom have to change primary jets or secondary rods more than one size and change only the hanger. I know there are very succesful foot brake cars and my hat is off to them. I tried it for 5 years and couldn't get it right.